In extracting hydrocarbons from the earth it is common to drill a borehole into the earth formation containing the hydrocarbons. A drill bit is attached to a drill string, including joined sections of drill pipe, suspended from a drilling rig. As the drill bit rotates, the hole deepens and the string is lengthened by attaching additional sections of drill pipe. During drilling operations, drilling fluid, or “mud” as it is also known, is pumped down through the drill pipe and into the hole through the drill bit. Drilling fluids are used to lubricate the drill-bit and keep it cool. The drilling mud also cleans the bit, and balances pressure by providing weight downhole, as well as bringing up to the surface sludge and cuttings created during the drilling process.
Slush or mud pumps are commonly used for pumping the drilling mud. Because of the need to pump the drilling mud through several thousand feet of drill pipe, such pumps typically operate at very high pressures. Moreover, it is necessary for the mud to emerge from the drill bit downhole at a relatively high velocity to lubricate and cool the bit and to effectively remove cuttings from the hole. Lastly, the fluid pressure generated by the mud pump contributes to maintaining a predetermined total downhole pressure, which is necessary in order to prevent dangerous and costly well blowouts.
The pistons and cylinders used for such mud pumps are susceptible to a high degree of wear during use because the drilling mud is relatively dense and has a high proportion of suspended abrasive solids. As the cylinder in which the piston reciprocates becomes worn, the small annular space between the piston head and the cylinder wall increases substantially and sometimes irregularly. This decreases the efficiency of the pump. To reduce the effect of this wear, the cylinder typically is provided with an expendable cylinder liner, which can be easily replaced.
It is the usual practice to replace the cylinder liner at end of its useful life. The pump cylinder liner in a duplex pump typically has an average life of 1200 to 1500 pump hours, or about 90 to 100 days. A duplex pump has two reciprocating pistons that each force fluid into a discharge line. The average life of the cylinder liners in a triplex pump is about 500 to 900 hours or about 50 to 60 days of service life at a normal duty cycle. Triplex reciprocating pumps have three pistons that force fluid into a discharge line. These fluid pumps can be single acting, in which fluid is discharged on alternate strokes, or double acting, in which each stroke discharges fluid.
In the course of installing or replacing a cylinder liner, the cylinder liner may become misaligned. Misaligned contact between the metal piston head and the cylinder creates considerable friction, abrasion, and heat. This, in turn, causes the cylinder liner, as well as other various pump parts, such as seals, to be susceptible to an increased rate of wear. In some cases, the frictional forces may even cause the seal to detach from the piston. For these reasons, the alignment of the cylinder liner of such pumps is critical.
Further, changing a cylinder liner in a mud pump is typically a difficult, dirty, and heavy job. Still further, because drilling rig time is very expensive, frequent replacement of cylinder liners causes considerable inconvenience if the system and apparatus for releasing the old cylinder liners and fitting the replacement cylinder liners are slow or difficult to operate. Thus, it is important that the system and method for aligning and securing the cylinder liners may be implemented without undue effort and down-time.
Some original pump designs include a large threaded “hammer nut” that is hammered on and off to hold the liner in place. Such a system for securing cylinder liners to respective pumping modules is difficult to operate with precision for a variety of reasons, including the involvement of heavy components, the handling of which may be dangerous for operators. These types of systems require considerable strength, skill and reliability of operators, together with the use of heavy tools in confined spaces. Thus, it is difficult to apply a specified torque to within a desired preset tolerance. Further, the securing force is dependent on the extent of wear and the general condition of the securing components.
There are several alternative ways to attach cylinder liners to their respective pumping modules, and these may vary according to make of pump in which they are used. One embodiment presently known employs a tapered concentric clamp, while another uses a concentric screw clamping arrangement. The tapered clamp is susceptible to corrosion and wear, which diminish its effectiveness. Other pump designs require large wrenches or impact socket tools to remove large nuts from studs so as to release the retainer. Not only is this not as precise way to load the liner seal, but in some models the rotation effect can dislodge and fail the seal mechanism. In all of these systems, the force securing the cylinder liner is difficult to control precisely, causing the cylinder liner to be susceptible to misalignment.
In still another known design, a replacement device involves removal of some of the original parts and uses hydraulics and Belleville washers to load, hold, and restrain the liner. This system relies on a spring lock, and therefore the securing force is dependent on the ability of the spring to retain its stiffness against the securing components. In addition, it relies on nuts secured on studs spaced about the circumference of the cylinder. Thus, this system causes the cylinder liner to be susceptible to misalignment arising from unequal securing forces at each stud, which can be caused by unequal tightening of each nut.
Accordingly, there remains a need to develop a new and improved system and apparatus for retaining and replacing a cylinder liner which overcomes certain of the foregoing difficulties while providing more advantageous overall results.